This question is bordering between aviation and biology. I am posting it here, since it is really about aerodynamics.

At first, one would think, that birds should be aerodynamically stable to save themselves the effort of maintaining stable flight.
However, even if humans had rigid joints they are in an unstable configuration other than four-legged animals. Thus, stability does not seem to be a must for biology, at least in certain aspects.

So, are birds aerodynamically stable? Are all of them, or are some species which are and some which aren't?

$\begingroup$They are not approximable with a rigid body, hence *dynamic stability is not really an applicable concept. You could ask about a non-flapping, wing-extended configuration, but even then I might have some reservations.$\endgroup$
– Federico♦May 23 '16 at 10:11

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$\begingroup$@Federico why does it have to be (approximately) a rigid body to be aerodynamically stable? I think you could argue if a bird can glide with all muscles relaxed, you could call it aerodynamically stable.$\endgroup$
– falstroMay 23 '16 at 10:27

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$\begingroup$Hmm... The on-topic page says that aerodynamics questions related to aircraft are on-topic here. Since a bird does not normally qualify as an aircraft, I'm inclined to agree with the off-topic close votes. Also, I agree with Federico in that you'd need to specify a particular bird and a particular configuration for this to be answerable regardless of what site it's on. You're probably more likely to find bird experts at Biology and the question should be on-topic there, too.$\endgroup$
– reirabMay 23 '16 at 14:00

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$\begingroup$I don't agree with the idea that this question is off-topic or more related to biology. Laws of aerodynamics apply equally to aircraft and birds. The question may be impossible to answer because this tight coupling of the aerodynamics and fly-by-wire control system in birds, but the same principles apply.$\endgroup$
– DeltaLimaMay 23 '16 at 14:12

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$\begingroup$Location and semantics aside, is the answer not terribly obvious for a vast majority of bird species?$\endgroup$
– Ryan MortensenMay 23 '16 at 15:38

1 Answer
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Birds have integrated active control systems that rely on several sensors to adjust the control surfaces to stabilize the flight. Since it is very difficult (perhaps impossible) to study the aerodynamic properties of the bird independently from its control system it is hard to make any claims on the static or dynamic stability of a bird.

What can be said is that from a control system point of view, most birds are observable, controllable and stabilizable in flight. This does not apply to the class of flightless birds.

If you are interested in the similarities in aerodynamics between insects, birds and aircraft I recommend getting a copy of "The Simple Science of Flight; from insects to jumbo jets" by Henk Tennekes, an Emeritus Professor of Aerospace Engineering at Pennsylvania State University.

After sitting in the garden on a sunny afternoon, observing various bird species and tinkering a bit more on the subject I actually think that many birds are unstable. Especially the smaller species that fly in the garden through bushes and trees, but also those that hunt for flying insects, are capable of extreme rapid changes in direction of flight; they are very agile.

There is a stark contrast between the behaviour of flying man-made
artifacts and the effortless elegance of bird flight. In this paper,
mechanisms are proposed that seems to be at least related to the way
birds fly when they are gliding (the no propulsion case). It is
recognized that biological structures and actuator systems (muscles)
are characterized by a great deal of compliance that produce a
completely different dynamic behaviour of flight. In this paper it is
demonstrated how this can be applied on a geometrically unstable
bird-like configuration. The result is that geometrically unstable
configurations can be stabilised without the need of rate gyros and/or
accelerometers. Although the bird might have a fast acting control
system for flight control, it is unlikely that it is fast enough for
active stabilisation of the short period oscillation, and this is
unnecessary with the model described here, which only requires
stabilisation of the long period oscillation.

In his introduction he also briefly refers to the work of Tennekes, which does not include much about the stability of birds. Also in the introduction he observes that birds have a cambered wing and must have an aft centre of gravity. This combination makes the bird unstable (if it were a rigid body).

$\begingroup$Hi DeltaLima, your garden observation put you on the right path. Birds are indeed unstable, in pitch, yaw and roll. The amount of instability depends on the wing planform and tail incidence, but in no wing position is a bird stable in yaw.$\endgroup$
– Peter KämpfMay 28 '16 at 7:45

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$\begingroup$For gliding, it should be possible to build a replica of the bird and study "the aerodynamic properties of the bird independently from its control system". I wonder if anybody tried.$\endgroup$
– h22May 28 '16 at 8:29